1. Field of the Invention
The present invention relates generally to a lithographic apparatus and a device manufacturing method.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In most lithography processes using a mask, a pellicle is used to protect the pattern surface of the mask from contamination, e.g., by dust particles. For exposure wavelengths down to 193 nm, the pellicle is conventionally a thin polymer film stretched over a frame fixed to the mask. The thin polymer pellicle has a negligible effect on imaging. However, it is at present not possible to make a thin polymer film that is sufficiently robust and transparent for use with exposure wavelengths of less than 193 nm, e.g., 157 nm. It is therefore proposed to use relatively thick pellicles at these wavelengths, e.g., quartz plates of thickness of several hundred Am, e.g., 300 or 800 .mu.m. Such a thick pellicle will adopt a non-planar shape when in use in a lithographic apparatus and having a different refractive index than the surrounding atmosphere has an optical power. Methods of minimizing the imaging effects of a thick pellicle and/or optimizing its shape are described in co-pending Application No. EP 04252493.4, which document is hereby incorporated by reference in its entirety. However, the present inventors have determined that there remain residual imaging effects due to the thick pellicle that are uncorrected by the methods described.